I also experienced the same error. I think I will like to state the procedures that I took to make mine to work, and then you can check to see if you got it wrong somewhere.
Here are the steps
First install the Gnome shell extensions package from the terminal
sudo apt install gnome-shell-extensions
And then reboot your system. After that, start GNOME Tweaks and you’ll find a few extensions installed. You can just toggle the button to start using an installed extension.
Next, go to GNOME Extensions website (GNOME User Themes Extensions) and download the extension with the latest version (Although, I often prefer to download the version next to the latest version, since it may be more stable).
Extract the downloaded file to the ~/.local/share/gnome-shell/extensions (home/.local/share/gnome-shell/extensions) directory. You can press Ctrl + H to show hidden folders. Create the extensions folder if you don't have it yet.
Now restart GNOME Shell. Press Alt+F2 and enter r to restart GNOME Shell.
Restart GNOME Tweaks tool as well. You should see the manually installed GNOME extension in the Tweak tool now. You can configure or enable the newly installed extension here.
That's all
I hope this helps